1. Field of the Invention
This invention relates to imaging in optical reading devices, and, more particularly, to actuator systems to automatically focus a target image in an optical reading device. Aspects of the invention are particularly useful in solid state, area image sensor based, handheld image readers which are positioned at variable orientations and distances with respect to a target image.
2. Description of the Related Art
Optical codes are patterns made up of image areas having different light reflective or light emissive properties, which are typically assembled in accordance with a priori rules. The term “barcode” is sometimes used to describe certain kinds of optical codes. The optical properties and patterns of optical codes are selected to distinguish them in appearance from the background environments in which they are used. Devices for identifying or extracting data from optical codes are sometimes referred to as “optical code readers” of which barcode scanners are one type. Optical code readers are used in both fixed and portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control and in transport vehicles for tracking package handling. The optical code can be used as a rapid, generalized means of data entry, for example, by reading a target barcode from a printed listing of many barcodes. In some uses, the optical code reader is connected to a portable data processing device or a data collection and transmission device. Frequently, the optical code reader includes a handheld sensor which is manually directed at a target code.
Most conventional optical scanning systems are designed to read one-dimensional barcode symbols. The barcode is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light reflecting characteristics. One example of a one dimensional barcode is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked barcode is the PDF417 barcode. A description of PDF417 barcode and techniques for decoding it are disclosed in U.S. Pat. No. 5,635,697 to Shellhammer et al., and assigned to Symbol Technologies, Inc., which is incorporated herein by reference. Another conventional optical code is known as “MaxiCode.” It consists of a central finder pattern or bull's eye center and a grid of hexagons surrounding the central finder. It should be noted that the aspects of the invention disclosed in this patent application are applicable to optical code readers, in general, without regard to the particular type of optical codes which they are adapted to read. The invention described herein is also applicable to image recognition and/or analysis devices.
Most conventional scanning systems generate one or more beams of laser light which reflects off a barcode symbol and back to the scanning system. The system obtains a continuous analog waveform corresponding to the light reflected by the code along one or more scan lines of the system. The system then decodes the waveform to extract information from the barcode. A system of this general type is disclosed, for example, in U.S. Pat. No. 4,251,798, assigned to Symbol Technologies, Inc. A beam scanning system for detecting and decoding one and two dimensional barcodes is disclosed in U.S. Pat. No. 5,561,283, also assigned to Symbol Technologies, Inc.
Barcodes can also be read employing imaging devices. For example an image sensor may be employed which has a two dimensional array of cells or photo sensors which correspond to image elements or pixels in a field of view of the device. Such an image sensor may be a two dimensional or area charge coupled device (CCD) and associated circuits for producing electronic signals corresponding to a two-dimensional array of pixel information for a field of view.
Many scanners in use today employ a scanning laser beam. Some such systems are deployed in handheld units which may be manually pointed at the target. Often an individual scanner is a component of a much larger system including other scanners, computers, cabling, data terminals, etc. Such systems are frequently designed and constructed on the basis of mechanical and optical specifications for the scanning engine, sometimes called “form factors”. One such form factor is the SE1200 form factor employed by Symbol Technologies, Inc.
Since current form factors specify scanning engines with smaller dimensions, there is a need to provide a compact imaging engine which can be substituted for conventional laser line scanning engines in currently designed and currently deployed optical code reader systems.
There is another need to provide an imaging engine which can be substituted for form factor scanning engines in currently designed and currently deployed optical code reading systems to increase the reliability, versatility and target working range of such systems.
It is known in the art to use a CCD photo detector and objective lens assembly in an optical code reader. In the past, such systems have employed complex objective lens assemblies originally designed for use in relatively expensive video imaging systems. Such lens assemblies typically employ multiple, large diameter, aspheric lens elements. Use of aspheric lens elements and a CCD photo detector in a code reader is illustrated in U.S. Pat. No. 5,703,349. Aspheric lens systems are relatively costly and difficult to build. They also have a single sharp focus and a limited depth of field, which along with conventional aiming, illumination and signal processing and decoding algorithms, limits the versatility and working range of the system.
Symbol Technologies, Inc. has developed bi-stable high speed zone collection systems for barcode scanners. These systems which employ lens structures moveable into the input optical path of the scanner (drop-in optics) are disclosed in U.S. Pat. Nos. 5,798,515 and 5,821,522.
Symbol Technologies, Inc. has also developed an easily constructed and inexpensive objective lens assembly for an imaging optical code reader. This assembly is disclosed in U.S. Pat. No. 6,340,114 B1, the contents of which are incorporated herein by reference. This patent also discloses an optical code reader which can be used to read codes at a wide range of distances. Additionally, this patent also discloses an imaging optical code reader with selectable fields of view and working depths of view appropriate to the signal processing and decoding capabilities of the reader.
However, notwithstanding the advancements made in the art, a need still exists for a system which will be small enough to comply with the most recent form factor specifications while offering the same or higher degree of versatility that may be found in existing systems.